1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device. More specifically, the invention relates to a method for forming a contact between wirings of a semiconductor device.
2. Description of the Related Art
In recent years, in accordance with the development of technology, miniaturization and performance of electronic devices has been advanced, and the highly developed electronic devices are everywhere. In order to maintain a company's competitive edge, it has become very important not only to produce better products than other companies, but also to offer such products at lower prices.
How to produce advanced electronic devices without increasing the cost becomes key; however, if reliability and quality are sacrificed, it is impossible to satisfy users' needs. Higher production yields is a very effective measure to achieve further price reduction without degrading performance, reliability, and quality.
Most of electronic devices used today are equipped with devices using semiconductor elements. Defects in the devices using semiconductor elements are some of the factors to cause lower yield. Defects due to disconnection and high resistivity of a wiring, and display defects and defects due to display unevenness in display devices can be given as defects in such semiconductor devices.
When forming a conductive film used for a wiring, disconnection and high resistivity of the wiring is occasionally caused by poor coverage of the conductive film. The poor coverage mostly occurs around an opening where the conductive film makes a contact with the lower layer. Further, the poor coverage can occur, for example, when the contact is not made since the opening is not covered with a wiring material, or when a part of a wiring becomes too thin.
Display defects and unevenness are often due to unevenness caused reflecting the shape of the opening for forming a contact with the lower layer or the shape of the conductive film in the lower layer. When the unevenness leads to nonuniformly of the electrical field, display unevenness are caused in a liquid crystal display device. Meanwhile in an EL device, display defects such as dark spots and bright spots would be caused as well as display unevenness.
As to an IC, a CPU, or the like, higher precision, higher integration, and multilevel interconnection have been achieved. However, depressions or projections in layers are overlapped with each other in repeating layering; so that a large step height would be formed (FIG. 2A). Such a step may cause further reduction in yield.
In order to reduce such defects and steps of an opening for forming a contact with the lower layer, a measure as disclosed in Reference 1 is taken, for example (Reference 1: Japanese Patent Laid-Open No. 7-37979).
There is no problem if the diameter of a contact hole is small; however, when the diameter of a contact hole 102 is large as shown in FIGS. 3A to 3C, there has been a problem that the depression could not be filled completely, and a step 105 would remain although reflow is performed.
Further, as shown in FIGS. 4A to 4C, the thickness of a wiring around the contact hole 102 becomes too thin and the thin portion 106 may have high resistance. Still further, when the wettability of a material of the wiring 104 with a base insulating film 101 is low, disconnection may occur in the worst case.
In order to prevent such high resistance and disconnection, an approach to increase the thickness of the wiring 104 can be conceived; however, excessive thickness is undesirable in view of integration. Accordingly, even a countermeasure such as etchback as in Reference 1 is taken, the accuracy of the etchback would cause a problem.